What is the module pattern and its evolution?

The module pattern encapsulates private state and exposes a public API. It evolved through several stages. Classic IIFE Module (pre-ES6): const counter = (function() { let count = 0; return { inc() { return ++count; }, reset() { count = 0; } }; })() — private count, public interface. Revealing Module: all logic defined privately, then selectively exposed — cleaner but can be confusing about what is public. CommonJS (Node.js): module.exports = { inc, reset } — synchronous, each file is a module with private scope. AMD (Asynchronous Module Definition): define(["dep"], function(dep) { return { } }) — async loading for browsers (RequireJS). UMD (Universal Module Definition): works in both CommonJS and AMD environments — used for libraries. ES Modules (ES6, current standard): export const inc = () => ++count; export default counter — static, tree-shakeable, async-loadable, native browser support. ES Modules are the definitive standard — all other patterns are legacy. Understanding the evolution helps when reading older codebases that use IIFE or CommonJS patterns.